feat(audio-cpp): scaffold native build contract

Pin and fetch audio.cpp, generate the LocalAI gRPC protocol sources, and propagate the upstream accelerator switches into the engine_runtime-linked server target.

Add a fixture-only contract test covering CPU, CUDA, Vulkan, Metal, legacy option rejection, and uname-based Darwin selection.

Assisted-by: Codex:gpt-5

backend/cpp/audio-cpp/Makefile

@@ -0,0 +1,58 @@
+# SPDX-License-Identifier: MIT
+
+AUDIO_CPP_VERSION?=f8fb0c19739193adfad0d9e58da99f25eda65256
+AUDIO_CPP_REPO?=https://github.com/0xShug0/audio.cpp
+AUDIO_CPP_SRC?=
+
+CURRENT_MAKEFILE_DIR := $(dir $(abspath $(lastword $(MAKEFILE_LIST))))
+BUILD_DIR := build
+
+BUILD_TYPE ?=
+JOBS ?= $(shell nproc 2>/dev/null || sysctl -n hw.ncpu 2>/dev/null || echo 4)
+UNAME_S := $(shell uname -s)
+
+CMAKE_ARGS ?= -DCMAKE_BUILD_TYPE=Release
+CMAKE_ARGS += -DENGINE_ENABLE_CUDA=OFF
+CMAKE_ARGS += -DENGINE_ENABLE_VULKAN=OFF
+CMAKE_ARGS += -DENGINE_ENABLE_METAL=OFF
+
+ifeq ($(BUILD_TYPE),cublas)
+	CMAKE_ARGS += -DENGINE_ENABLE_CUDA=ON
+else ifeq ($(BUILD_TYPE),vulkan)
+	CMAKE_ARGS += -DENGINE_ENABLE_VULKAN=ON
+else ifeq ($(UNAME_S),Darwin)
+	CMAKE_ARGS += -DENGINE_ENABLE_METAL=ON
+endif
+
+.PHONY: all grpc-server test clean purge
+
+all: grpc-server
+
+audio.cpp:
+ifneq ($(AUDIO_CPP_SRC),)
+	ln -sfn $(abspath $(AUDIO_CPP_SRC)) audio.cpp
+else
+	mkdir -p audio.cpp
+	cd audio.cpp && \
+	git init -q && \
+	git remote add origin $(AUDIO_CPP_REPO) && \
+	git fetch --depth 1 origin $(AUDIO_CPP_VERSION) && \
+	git checkout --detach FETCH_HEAD && \
+	git submodule update --init --recursive --depth 1
+endif
+
+grpc-server: audio.cpp
+	mkdir -p $(BUILD_DIR)
+	cd $(BUILD_DIR) && cmake $(CMAKE_ARGS) $(CURRENT_MAKEFILE_DIR)
+	cmake --build $(BUILD_DIR) --config Release \
+		--target audio-cpp-grpc-server -j $(JOBS)
+	cp $(BUILD_DIR)/audio-cpp-grpc-server grpc-server
+
+test:
+	bash tests/build_contract_test.sh
+
+clean:
+	rm -rf $(BUILD_DIR) grpc-server
+
+purge: clean
+	rm -rf audio.cpp